399 lines
14 KiB
ReStructuredText
399 lines
14 KiB
ReStructuredText
=================
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SanitizerCoverage
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=================
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.. contents::
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:local:
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Introduction
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============
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Sanitizer tools have a very simple code coverage tool built in. It allows to
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get function-level, basic-block-level, and edge-level coverage at a very low
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cost.
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How to build and run
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====================
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SanitizerCoverage can be used with :doc:`AddressSanitizer`,
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:doc:`LeakSanitizer`, :doc:`MemorySanitizer`,
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UndefinedBehaviorSanitizer, or without any sanitizer. Pass one of the
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following compile-time flags:
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* ``-fsanitize-coverage=func`` for function-level coverage (very fast).
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* ``-fsanitize-coverage=bb`` for basic-block-level coverage (may add up to 30%
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**extra** slowdown).
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* ``-fsanitize-coverage=edge`` for edge-level coverage (up to 40% slowdown).
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At run time, pass ``coverage=1`` in ``ASAN_OPTIONS``,
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``LSAN_OPTIONS``, ``MSAN_OPTIONS`` or ``UBSAN_OPTIONS``, as
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appropriate. For the standalone coverage mode, use ``UBSAN_OPTIONS``.
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Example:
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.. code-block:: console
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% cat -n cov.cc
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1 #include <stdio.h>
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2 __attribute__((noinline))
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3 void foo() { printf("foo\n"); }
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4
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5 int main(int argc, char **argv) {
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6 if (argc == 2)
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7 foo();
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8 printf("main\n");
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9 }
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% clang++ -g cov.cc -fsanitize=address -fsanitize-coverage=func
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% ASAN_OPTIONS=coverage=1 ./a.out; ls -l *sancov
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main
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-rw-r----- 1 kcc eng 4 Nov 27 12:21 a.out.22673.sancov
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% ASAN_OPTIONS=coverage=1 ./a.out foo ; ls -l *sancov
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foo
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main
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-rw-r----- 1 kcc eng 4 Nov 27 12:21 a.out.22673.sancov
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-rw-r----- 1 kcc eng 8 Nov 27 12:21 a.out.22679.sancov
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Every time you run an executable instrumented with SanitizerCoverage
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one ``*.sancov`` file is created during the process shutdown.
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If the executable is dynamically linked against instrumented DSOs,
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one ``*.sancov`` file will be also created for every DSO.
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Postprocessing
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==============
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The format of ``*.sancov`` files is very simple: the first 8 bytes is the magic,
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one of ``0xC0BFFFFFFFFFFF64`` and ``0xC0BFFFFFFFFFFF32``. The last byte of the
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magic defines the size of the following offsets. The rest of the data is the
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offsets in the corresponding binary/DSO that were executed during the run.
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A simple script
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``$LLVM/projects/compiler-rt/lib/sanitizer_common/scripts/sancov.py`` is
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provided to dump these offsets.
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.. code-block:: console
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% sancov.py print a.out.22679.sancov a.out.22673.sancov
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sancov.py: read 2 PCs from a.out.22679.sancov
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sancov.py: read 1 PCs from a.out.22673.sancov
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sancov.py: 2 files merged; 2 PCs total
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0x465250
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0x4652a0
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You can then filter the output of ``sancov.py`` through ``addr2line --exe
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ObjectFile`` or ``llvm-symbolizer --obj ObjectFile`` to get file names and line
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numbers:
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.. code-block:: console
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% sancov.py print a.out.22679.sancov a.out.22673.sancov 2> /dev/null | llvm-symbolizer --obj a.out
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cov.cc:3
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cov.cc:5
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Sancov Tool
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===========
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A new experimental ``sancov`` tool is developed to process coverage files.
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The tool is part of LLVM project and is currently supported only on Linux.
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It can handle symbolization tasks autonomously without any extra support
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from the environment. You need to pass .sancov files (named
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``<module_name>.<pid>.sancov`` and paths to all corresponding binary elf files.
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Sancov matches these files using module names and binaries file names.
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.. code-block:: console
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USAGE: sancov [options] <action> (<binary file>|<.sancov file>)...
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Action (required)
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-print - Print coverage addresses
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-covered-functions - Print all covered functions.
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-not-covered-functions - Print all not covered functions.
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-symbolize - Symbolizes the report.
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Options
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-blacklist=<string> - Blacklist file (sanitizer blacklist format).
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-demangle - Print demangled function name.
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-strip_path_prefix=<string> - Strip this prefix from file paths in reports
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Coverage Reports (Experimental)
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================================
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``.sancov`` files do not contain enough information to generate a source-level
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coverage report. The missing information is contained
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in debug info of the binary. Thus the ``.sancov`` has to be symbolized
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to produce a ``.symcov`` file first:
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.. code-block:: console
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sancov -symbolize my_program.123.sancov my_program > my_program.123.symcov
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The ``.symcov`` file can be browsed overlayed over the source code by
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running ``tools/sancov/coverage-report-server.py`` script that will start
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an HTTP server.
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How good is the coverage?
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=========================
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It is possible to find out which PCs are not covered, by subtracting the covered
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set from the set of all instrumented PCs. The latter can be obtained by listing
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all callsites of ``__sanitizer_cov()`` in the binary. On Linux, ``sancov.py``
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can do this for you. Just supply the path to binary and a list of covered PCs:
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.. code-block:: console
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% sancov.py print a.out.12345.sancov > covered.txt
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sancov.py: read 2 64-bit PCs from a.out.12345.sancov
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sancov.py: 1 file merged; 2 PCs total
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% sancov.py missing a.out < covered.txt
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sancov.py: found 3 instrumented PCs in a.out
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sancov.py: read 2 PCs from stdin
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sancov.py: 1 PCs missing from coverage
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0x4cc61c
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Edge coverage
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=============
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Consider this code:
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.. code-block:: c++
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void foo(int *a) {
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if (a)
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*a = 0;
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}
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It contains 3 basic blocks, let's name them A, B, C:
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.. code-block:: none
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A
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C
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If blocks A, B, and C are all covered we know for certain that the edges A=>B
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and B=>C were executed, but we still don't know if the edge A=>C was executed.
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Such edges of control flow graph are called
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`critical <http://en.wikipedia.org/wiki/Control_flow_graph#Special_edges>`_. The
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edge-level coverage (``-fsanitize-coverage=edge``) simply splits all critical
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edges by introducing new dummy blocks and then instruments those blocks:
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.. code-block:: none
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A
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D B
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C
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Tracing PCs
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===========
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*Experimental* feature similar to tracing basic blocks, but with a different API.
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With ``-fsanitize-coverage=trace-pc`` the compiler will insert
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``__sanitizer_cov_trace_pc()`` on every edge.
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With an additional ``...=trace-pc,indirect-calls`` flag
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``__sanitizer_cov_trace_pc_indirect(void *callee)`` will be inserted on every indirect call.
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These callbacks are not implemented in the Sanitizer run-time and should be defined
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by the user. So, these flags do not require the other sanitizer to be used.
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This mechanism is used for fuzzing the Linux kernel (https://github.com/google/syzkaller)
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and can be used with `AFL <http://lcamtuf.coredump.cx/afl>`__.
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Tracing PCs with guards
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=======================
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With ``-fsanitize-coverage=trace-pc-guard`` the compiler will insert the following code
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on every edge:
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.. code-block:: none
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__sanitizer_cov_trace_pc_guard(&guard_variable)
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Every edge will have its own `guard_variable` (uint32_t).
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The compler will also insert a module constructor that will call
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.. code-block:: c++
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// The guards are [start, stop).
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// This function will be called at least once per DSO and may be called
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// more than once with the same values of start/stop.
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__sanitizer_cov_trace_pc_guard_init(uint32_t *start, uint32_t *stop);
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With `trace-pc-guards,indirect-calls`
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``__sanitizer_cov_trace_pc_indirect(void *callee)`` will be inserted on every indirect call.
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The functions `__sanitizer_cov_trace_pc_*` should be defined by the user.
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Example:
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.. code-block:: c++
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// trace-pc-guard-cb.cc
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#include <stdint.h>
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#include <stdio.h>
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#include <sanitizer/coverage_interface.h>
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// This callback is inserted by the compiler as a module constructor
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// into every DSO. 'start' and 'stop' correspond to the
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// beginning and end of the section with the guards for the entire
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// binary (executable or DSO). The callback will be called at least
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// once per DSO and may be called multiple times with the same parameters.
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extern "C" void __sanitizer_cov_trace_pc_guard_init(uint32_t *start,
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uint32_t *stop) {
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static uint64_t N; // Counter for the guards.
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if (start == stop || *start) return; // Initialize only once.
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printf("INIT: %p %p\n", start, stop);
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for (uint32_t *x = start; x < stop; x++)
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*x = ++N; // Guards should start from 1.
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}
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// This callback is inserted by the compiler on every edge in the
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// control flow (some optimizations apply).
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// Typically, the compiler will emit the code like this:
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// if(*guard)
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// __sanitizer_cov_trace_pc_guard(guard);
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// But for large functions it will emit a simple call:
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// __sanitizer_cov_trace_pc_guard(guard);
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extern "C" void __sanitizer_cov_trace_pc_guard(uint32_t *guard) {
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if (!*guard) return; // Duplicate the guard check.
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// If you set *guard to 0 this code will not be called again for this edge.
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// Now you can get the PC and do whatever you want:
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// store it somewhere or symbolize it and print right away.
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// The values of `*guard` are as you set them in
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// __sanitizer_cov_trace_pc_guard_init and so you can make them consecutive
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// and use them to dereference an array or a bit vector.
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void *PC = __builtin_return_address(0);
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char PcDescr[1024];
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// This function is a part of the sanitizer run-time.
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// To use it, link with AddressSanitizer or other sanitizer.
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__sanitizer_symbolize_pc(PC, "%p %F %L", PcDescr, sizeof(PcDescr));
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printf("guard: %p %x PC %s\n", guard, *guard, PcDescr);
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}
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.. code-block:: c++
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// trace-pc-guard-example.cc
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void foo() { }
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int main(int argc, char **argv) {
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if (argc > 1) foo();
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}
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.. code-block:: console
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clang++ -g -fsanitize-coverage=trace-pc-guard trace-pc-guard-example.cc -c
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clang++ trace-pc-guard-cb.cc trace-pc-guard-example.o -fsanitize=address
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ASAN_OPTIONS=strip_path_prefix=`pwd`/ ./a.out
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.. code-block:: console
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INIT: 0x71bcd0 0x71bce0
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guard: 0x71bcd4 2 PC 0x4ecd5b in main trace-pc-guard-example.cc:2
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guard: 0x71bcd8 3 PC 0x4ecd9e in main trace-pc-guard-example.cc:3:7
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.. code-block:: console
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ASAN_OPTIONS=strip_path_prefix=`pwd`/ ./a.out with-foo
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.. code-block:: console
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INIT: 0x71bcd0 0x71bce0
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guard: 0x71bcd4 2 PC 0x4ecd5b in main trace-pc-guard-example.cc:3
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guard: 0x71bcdc 4 PC 0x4ecdc7 in main trace-pc-guard-example.cc:4:17
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guard: 0x71bcd0 1 PC 0x4ecd20 in foo() trace-pc-guard-example.cc:2:14
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Tracing data flow
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=================
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Support for data-flow-guided fuzzing.
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With ``-fsanitize-coverage=trace-cmp`` the compiler will insert extra instrumentation
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around comparison instructions and switch statements.
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Similarly, with ``-fsanitize-coverage=trace-div`` the compiler will instrument
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integer division instructions (to capture the right argument of division)
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and with ``-fsanitize-coverage=trace-gep`` --
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the `LLVM GEP instructions <http://llvm.org/docs/GetElementPtr.html>`_
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(to capture array indices).
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.. code-block:: c++
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// Called before a comparison instruction.
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// Arg1 and Arg2 are arguments of the comparison.
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void __sanitizer_cov_trace_cmp1(uint8_t Arg1, uint8_t Arg2);
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void __sanitizer_cov_trace_cmp2(uint16_t Arg1, uint16_t Arg2);
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void __sanitizer_cov_trace_cmp4(uint32_t Arg1, uint32_t Arg2);
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void __sanitizer_cov_trace_cmp8(uint64_t Arg1, uint64_t Arg2);
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// Called before a switch statement.
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// Val is the switch operand.
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// Cases[0] is the number of case constants.
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// Cases[1] is the size of Val in bits.
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// Cases[2:] are the case constants.
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void __sanitizer_cov_trace_switch(uint64_t Val, uint64_t *Cases);
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// Called before a division statement.
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// Val is the second argument of division.
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void __sanitizer_cov_trace_div4(uint32_t Val);
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void __sanitizer_cov_trace_div8(uint64_t Val);
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// Called before a GetElemementPtr (GEP) instruction
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// for every non-constant array index.
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void __sanitizer_cov_trace_gep(uintptr_t Idx);
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This interface is a subject to change.
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The current implementation is not thread-safe and thus can be safely used only for single-threaded targets.
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Output directory
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================
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By default, .sancov files are created in the current working directory.
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This can be changed with ``ASAN_OPTIONS=coverage_dir=/path``:
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.. code-block:: console
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% ASAN_OPTIONS="coverage=1:coverage_dir=/tmp/cov" ./a.out foo
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% ls -l /tmp/cov/*sancov
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-rw-r----- 1 kcc eng 4 Nov 27 12:21 a.out.22673.sancov
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-rw-r----- 1 kcc eng 8 Nov 27 12:21 a.out.22679.sancov
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Sudden death
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============
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Normally, coverage data is collected in memory and saved to disk when the
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program exits (with an ``atexit()`` handler), when a SIGSEGV is caught, or when
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``__sanitizer_cov_dump()`` is called.
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If the program ends with a signal that ASan does not handle (or can not handle
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at all, like SIGKILL), coverage data will be lost. This is a big problem on
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Android, where SIGKILL is a normal way of evicting applications from memory.
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With ``ASAN_OPTIONS=coverage=1:coverage_direct=1`` coverage data is written to a
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memory-mapped file as soon as it collected.
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.. code-block:: console
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% ASAN_OPTIONS="coverage=1:coverage_direct=1" ./a.out
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main
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% ls
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7036.sancov.map 7036.sancov.raw a.out
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% sancov.py rawunpack 7036.sancov.raw
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sancov.py: reading map 7036.sancov.map
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sancov.py: unpacking 7036.sancov.raw
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writing 1 PCs to a.out.7036.sancov
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% sancov.py print a.out.7036.sancov
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sancov.py: read 1 PCs from a.out.7036.sancov
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sancov.py: 1 files merged; 1 PCs total
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0x4b2bae
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Note that on 64-bit platforms, this method writes 2x more data than the default,
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because it stores full PC values instead of 32-bit offsets.
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